Facilities of this kind are needed in computer systems where information having safety implications is processed by several computers in parallel, and where the results are delivered only if received by the facility in identical form from a majority of the computers.
For example, DE-OS No. 21 08 496 (particularly FIG. 2) discloses a circuit arrangement in which the outputs from three computers handling the information in parallel are compared in pairs by comparator circuits. The results of the comparisons are fed to a control circuit which makes a majority decision. According to the results of the majority decision, a switching facility is so controlled that only that information is output onto communication channels which was produced by a majority of the computers. The outputs from the comparator circuits and the control circuit are fed back to the computers, which can thus monitor the operation of these circuits. Such a circuit arrangement is suitable for applications in which only few outputs, e.g., serial data telegram outputs, are needed to deliver the results produced by the computers. In such circuit arrangements, the amount of circuitry required for comparators and switching facilities is approximately proportional to the number of outputs required, so that in applications with large numbers of outputs, e.g., if microcomputers are employed, the costs of a comparator and majority-voting logic circuit designed in the manner of the circuit arrangement described in DE-OS No. 21 08 496 would be prohibitive.
It is also known (see, for example, DE-OS No. 30 09 355, particularly page 3) to have the comparison necessary prior to each majority decision performed by the computers themselves. In that case, the computers exchange their results or, as described in DE-OS No. 30 09 355, reduced amounts of data derived from these results. The prior publication shows neither how a majority decision can be made with three computers nor how the results produced by the majority of the computers can be output in a safe manner.
The object of the invention is to provide a 2-out-of-3 selecting facility of this kind which permits a majority decision and a fail-safe output of the result produced by the majority of the computers without the need for expensive, fail-safe circuit components. It is described by the features of claim 1.
The selecting facility according to the invention is especially suited for use together with microcomputers having a large number of outputs. As the results produced by the computers need no longer be compared in pairs outside the computers, no comparator circuits with large numbers of components are required. The transfer switches switch the output buses rather than the individual outputs, so that their design no longer depends on the number of outputs.
A development of the selecting facility according to the invention relates to a majority voting circuit suitable for a microcomputer system.
A further development concerns the assignment of the computer outputs to the transfer switches. With this development, in conjunction with special operations performed on the results appearing at the output ports, maximum safety is achieved using a particularly simple transfer-switch configuration.